I have AEM 6.5.8 version and after the creation of an Experience Fragments Variation as live-copy (standard rollout or push on modify), i'm not able to break the inheritance at page property level. As touch UI, i don't see the CTA to break the inheritance of the property (as in the screenshot of documentation), but the fields can be modified as per screenshot. Anyway even if i change a property and after that i modify the master variation, after the rollout the live copy variation property will be overridden with master one (losing the information).
NOTE: i want highlight that this behaviour occurs only for experience fragments variation (sling:resourceSuperType cq/experience-fragments/components/xfpage)
Unlike sites, Locking -> Disabling/cancel inheritance functionality at page properties level is not available OOTB with respect to Experience Fragments Live copy variation.
This is evident by the fact that there is no mixin named "mix:lockable" on live copy variation(live copy xf page's jcr:content) along with cq:LiveRelationship nor presence of cq-msm-lockable property for each of the properties of xfpage component's dialog - /libs/cq/experience-fragments/components/xfpage/cq:dialog